net: ethernet: mtk_eth_soc: missing mutex

Patch 2ed37183 ("netfilter: flowtable: separate replace, destroy and
stats to different workqueues") splits the workqueue per event type. Add
a mutex to serialize updates.

Fixes: 502e84e2

 ("net: ethernet: mtk_eth_soc: add flow offloading support")
Reported-by: Frank Wunderlich <frank-w@public-files.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>